With the development of technology, wearable display devices have been widely used in video, film, medicine, simulation training, games and other fields. In recent years, a head-mounted display (HMD) configured for Virtual Reality (VR) or Augmented Reality (AR) has been developed. Generally, a conventional wearable display device configured for virtual reality includes a casing, a flat-panel display and a convex lens. The convex lens converts an image of the flat-panel display into a stand upright virtual image. When wearing the wearable display device, a user can receive various kinds of sensory simulations provided by the computers or the electronic devices through a specific media and experience a virtual world.